Experimental and theoretical investigation on axial compressive bearing capacity of timber- and concrete-filled square steel tubes

Traditional concrete-filled steel tubes (CFST) are widely used for their strength and ductility; however, they add considerable weight and rely on resource-intensive materials, which limits their sustainability and efficiency. With increasing demand for lighter, more sustainable, and energy-efficient construction solutions, timber presents a renewable alternative. Its lightweight and high energy absorption capacity make it an attractive option for reducing structural weight while maintaining or enhancing performance. This study investigates the behavior of timber- and concrete-filled square steel tubes (TCFST) under axial compressive loading, with the aim of improving both structural efficiency and material utilization. A comprehensive experimental investigation was conducted using 19 specimens, including four types of TCFST columns with varying combinations of timber and concrete infill materials within square steel tubes. The study analyzes the load-bearing capacity, deformation characteristics, failure modes, stiffness, and energy absorption capabilities of the specimens under axial compression. The results reveal that different hybrid infill materials and configurations significantly affect mechanical performance, such as load capacity, stiffness, and energy absorption. The results indicate that columns with higher timber content generally exhibit greater the axial compressive capacity ratio to weight (ACRW) and total absorbed energy ratio to weight (EDRW), suggesting that the incorporation of timber enhances energy absorption capabilities and structural efficiency. Key findings include the linear relationship between axial capacity and timber content, and the influence of timber content on the energy absorption and failure mechanisms of TCFST columns. Results showed that axial capacity decreased linearly with increasing timber content, with up to 22.6 % reduction compared to concrete-filled tubes. However, specimens with higher timber ratios exhibited superior efficiency: the axial compressive capacity ratio to weight (ACRW) rose significantly, reaching over 150 kN/kg for Type-IV columns, nearly double that of concrete-filled references. Energy absorption also improved, with maximum absorbed energy of 4.36 kJ in C-T-S-S90 (9 % higher than concrete-filled specimens). Timber-filled square steel tubes offer reduced structural weight and enhanced energy absorption, making them promising for applications requiring materials that can withstand significant energy under loading.